British Prime Minister Theresa May has given Vladimir Putin’s administration until midnight on Tuesday to explain how a former Russian spy was poisioned in Salisbury.

The PM said that Russia must explain how the Russian-made nerve agent was used to poison Sergei Skripal and his daughter, or else, she will conclude it was an “unlawful use of force” by the Russian state against the UK.

The PM said it was "highly likely" Russia was responsible for the attack in Salisbury, Wiltshire, last Sunday.

After chairing a meeting of the national security council, she warned that Britain would not tolerate such a “brazen attempt to murder innocent civilians on our soil”.

Describing the incident as an “indiscriminate and reckless act”, she said that the foreign secretary, Boris Johnson, had summoned the Russian ambassador to Whitehall and demanded an explanation by the end of Tuesday.

May also spoke to French President Emmanuel Macron on Monday and the two leaders "agreed that it would be important to continue to act in concert with allies" to address what it called "the wide pattern of aggressive Russian behaviour", her spokesman said.

Russian officials immediately hit back, with Maria Zakharova, the spokeswoman for the Russian foreign minister, calling the remarks “a provocation” and describing the event as a “circus show in the British parliament”.

Yevgeny Serebrennikov, first deputy chair of the Russian Federation Council's Defence and Security Committee, described Mrs May's accusations as "unfounded".

“We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses."

He described it as a "really egregious act" and said he had become "extremely concerned" by Russia becoming "more aggressive".

Former double agent Sergei Skripal, 66, and his daughter, Yulia, 33, were found on a bench in Salisbury city centre. They remain in a critical but stable condition in hospital.

Det Sgt Nick Bailey, who fell ill attending to the pair, remains seriously ill, but has been talking to his family.

May told the Commons on Monday that the poison used in the attack was Novichok - a military-grade nerve agent developed by Russia.

"Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," she said.